Humidity sensors are utilized in a variety of sensing applications. Humidity sensors can be implemented in the context of semiconductor-based sensors utilized in many industrial applications. Solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors, for example, are fabricated using semiconductor processes.
Many modern processes, for example, require measurement of relative humidity at dew points between −40° C. and 180° C., corresponding to relative humidity between 1% and 100%. There is a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres.
Humidity can be measured by a number of techniques. In a semiconductor-based system, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be sensed by electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer.
Two of the most common physical changes are the change in resistance and the change in dielectric constant, which can be respectively translated into a resistance change or a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result is inaccuracy or erroneous readings, among other problems.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid the problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
Humidity-sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting substrate structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time. Capacitive humidity sensors are typically constructed by depositing several layers of material on a substrate material. An example of a humidity sensor is disclosed in U.S. Pat. No. 6,724,612, entitled “Relative Humidity Sensor with Integrated Signal Conditioning,” which issued to Davis et al on Apr. 20, 2004, and issued to Honeywell International, Inc. U.S. Pat. No. 6,724,612 is incorporated herein by reference.
One of the problems with conventional humidity sensors is that such devices are continually plagued with inaccurate output due to disruption of the sensing polymer from condensation. A need exists for configuring and providing heated humidity sensor structures with components of similar material construction and size for improved heat conduction thus resulting low power heating capability. It is believed that the embodiments disclosed herein provide for such capabilities.